The application claims a priority based on Japanese Patent Application Nos. 2000-59345 and 2001-53801 filed on Mar. 3, 2000 and Feb. 28, 2001, respectively, the entire contents of which are incorporated herein by reference for all purposes.
1. Field of the Invention
The present invention relates to a playback apparatus for a multilayer-recording optical disk. More particularly, it relates to layer jump control technology wherein an object lens is moved in its focusing direction in order to perform a focusing servo operation for any desired recording layer.
2. Description of the Prior Art
In recent years, an optical disk called xe2x80x9cDVDxe2x80x9d has come into practical use as a recording medium of large storage capacity. The DVD has at most two recording layers per side, and data can be recorded on both the sides thereof. The function of controlling the movement of an object lens in the focusing direction thereof (xe2x80x9clayer jump functionxe2x80x9d) is required of a playback apparatus for such a multilayer-recording optical disk in order that, when the playback of one recording layer (simply termed xe2x80x9clayerxe2x80x9d below) has been requested in a state where a focusing servo operation is proceeding for the other layer under playback, a focusing servo operation may be performed for the desired layer. A multilayer-recording optical disk playback apparatus in the prior art incarnates the above function by processing as stated below.
FIG. 12 is a flow chart of a layer jump process in the case where, during the playback of the lower layer of an optical disk which includes the two recording layers of the lower layer (layer nearer to the object lens, and termed xe2x80x9clayer 0xe2x80x9d) and an upper layer (termed xe2x80x9clayer 1xe2x80x9d), the playback of the upper layer has been requested. FIG. 13 is a timing chart showing the relationship among a focus error signal, control signals, etc. on this occasion. Referring to FIG. 13, FcH and FcL comparator slice levels are reference voltages with which the focus error signal is compared and whose values are set at the shipment of the playback apparatus beforehand. The FcH signal assumes a Hi (high) output during a time period for which the voltage of the focus error signal exceeds the FcH comparator slice level (the voltage goes Hi), whereas it assumes a Lo (low) output during any other time period. On the other hand, the FcL signal assumes the Hi output during a time period for which the voltage of the focus error signal exceeds the FcL comparator slice level (the voltage goes Lo), whereas it assumes the Lo output during any other time period.
A coil portion is disposed around an object lens which condenses a laser beam on the recording layer of the optical disk, and they are supported by a spring so as to be ascendible and descendible. When a kick voltage is applied to the coil, a force is exerted in the direction of bringing the object lens near to the optical disk. In contrast, when a brake voltage is applied, a force is exerted in the direction of bringing the object lens away from the optical disk.
When the playback of the layer 1 is requested during the playback of the layer 0, that is, in a state where a focusing servo operation is proceeding for the layer 0, the playback apparatus turns OFF a focusing servo (S401), whereupon it applies the kick voltage in the direction in which the object lens ascends (that is, in which the object lens comes near to the optical disk) (S402, time a in FIG. 13). Thereafter, it monitors a course from the rise of the pulse FcL (time b in FIG. 13) till the fall thereof (time c in FIG. 13) (S403). Upon detecting the fall of the pulse FcL, it terminates the application of the kick voltage (S404). Thereafter, it begins to monitor the rise of the pulse FcH (time d in FIG. 13) (S405). Upon detecting the rise of the pulse FcH (time d in FIG. 13), it applies the brake voltage in the direction in which the object lens descends (S406). Thereafter, it begins to monitor the fall of the pulse FcH (S407). Upon detecting the fall of the pulse FcH (time e in FIG. 13), it stops the application of the brake voltage (S408). Subsequently, it turns ON the focusing servo (S409), thereby to start the playback of the layer 1. Incidentally, control processing in a tracking direction is omitted here.
In the process explained above, the FcH and FcL comparator slice levels have the preset constant values. Therefore, the layer jump function cannot cope with discrepancy in the error levels of individual optical disks or playback apparatuses, or changes in the characteristics of the playback apparatus attributed to ambient conditions such as temperature. Moreover, since the comparator slice levels need to be confined within the peak levels of the focus error signal reliably in any playback state, they cannot be set at very large values. Therefore, in case of the occurrence of a focus error phenomenon called xe2x80x9cstray lightxe2x80x9d wherein a small peak different from the essential peaks of the focus error signal appears in the vicinity of the reference level thereof, the layer jump might end in failure due to the misrecognition of a peak point, depending upon the values of the comparator slice levels.
An object of the invention is to provide a layer jump technique of high stability which can cope with discrepancy in the error levels of individual optical disks or playback apparatuses, and changes in the characteristics of the playback apparatus attributed to ambient conditions such as temperature.
In order to accomplish the above object, a focusing control device according to the invention is characterized by comprising an object lens which condenses a light beam on an optical disk constructed of a plurality of signal recording layers; focusing drive means for moving said object lens in a direction orthogonal to the recording layers of the optical disk; photodetection means for detecting light reflected from said optical disk; focus error signal generation means for generating a focus error signal which corresponds to defocusing of said object lens relative to any of said recording layers of said optical disk, on the basis of a detection signal of said photodetection means; peak detection means for detecting peaks of the focus error signal; reference value calculation means for calculating reference values of said focus error signal in accordance with detection signals of said peak detection means; comparison signal generation means for comparing said focus error signal with its reference values, and generating comparison signals based on results of the comparisons; and control means for accepting a request for moving a focusing position of said object lens, and generating and outputting signals which control said focusing drive means, on the basis of the detection signals of said peak detection means and the comparison signals.